A delta-sigma modulator or sigma-delta modulator is a feedback system that employs simple circuit blocks to achieve high-resolution output signals. The delta-sigma modulator is widely adapted to electronics circuits such as analog-to-digital converters (A/D), digital-to-analog converters (D/A) or frequency synthesizers, and is highly attractive in wireless communications due to its simplicity and low-power consumption. The radio-frequency (RF) bandpass delta-sigma modulator is one type of delta-sigma modulator that is well adaptable to RF signal reception and subsequent digitization without frequency down conversion.
Advancement in semiconductor process technologies allows usage of the quadrature bandpass-sampling delta-sigma analog-to-digital demodulator (QBS-ADD) at RF frequencies, which is a new promising low-cost and reliable technique to digitize RF signals directly at the antenna. The technology combines the bandpass sampling theory as a direct-conversion technique to demodulate RF signals down to baseband with the continuous-time bandpass delta-sigma technique to increase the A/D conversion resolution.
Because the QBS-ADD is based on the quadrature direct-conversion technique, a phase mismatch in the in-phase (I) and quadrature (Q) sampling clocks can corrupt the demodulated in-phase and quadrature components. A I/Q calibration technique discussed in disclosed embodiments is intended to correct the I/Q phase mismatch based on quantifying the phase error from the demodulated in-phase and quadrature components in the frequency domain.